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vapor permeation process
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Microporous and Mesoporous Materials 115 (2008) 184–188 www.elsevier.com/locate/micromeso
Dehydrating performance of commercial LTA zeolite membranes and application to fuel grade bio-ethanol production by hybrid distillation/vapor permeation process Kiminori Sato *, Katsuhiko Aoki, Kazunori Sugimoto, Kou Izumi, Soushi Inoue, Junji Saito, Shiro Ikeda, Takashi Nakane Bussan Nanotech Research Institute Inc., Koyadai 2-1, 305-0074 Tsukuba, Japan Received 19 July 2007; accepted 24 October 2007 Available online 19 February 2008
Abstract The performance of commercial NaA zeolite membranes (Bussan Nanotech Research Institute Inc., Japan) was investigated for dehydration viability by vapor permeation (VP) to apply them to production of fuel grade ethanol. We carried out mainly two sub-projects: (1) the VP experiments with a large unit to determine the dehydration performance for elemental membranes and (2) the field test to produce fuel grade ethanol from fermented hydrous ethanol (5–10 wt.% ethanol content) through hybrid distillation/vapor permeation system in which the membrane modules were installed. The commercial NaA zeolite membranes exhibited sufficiently high dehydrating performance for economically feasible fluxes. It has been also verified that anhydrous ethanol (>99.7 wt.%) with an amount of 46 kg h 1 can be continuously produced with the commercial NaA zeolite membranes in a pilot plant. These results suggest that the commercial NaA zeolite membrane can be expected to be utilized in motor fuel ethanol production. Ó 2008 Published by Elsevier Inc. Keywords: LTA; Zeolite membrane; Bio-ethanol; Dehydration; Vapor permeation
1. Introduction Ethanol production for motor fuel grade is rapidly growing worldwide, because it has been recommended by governments to utilize bio-ethanol as an additive to gasoline for reduction of fossil energy consumption and of CO2 emission. Many kinds of feedstock are fermented to produce hydrous ethanol that has to be purified finally to the anhydrous ethanol (>99.2 wt.%) by dehydration for motor grade fuel. Distillation is a dehydration process for hydrous ethanol, but it is well known that simple distillation process cannot purify to anhydrous ethanol because there is the azeotrope with water(4 wt.%)/ethanol(96 wt.%) through which more anhydrous ethanol cannot be produced. The azeotropic distillation by additional component *
Corresponding author. Tel.: +81 29 839 9371; fax: +81 29 29 9430. E-mail address: [email protected] (K. Sato).
1387-1811/$ - see front matter Ó 2008 Published by Elsevier Inc. doi:10.1016/j.micromeso.2007.10.053
is one way to break the azeotrope for purification of motor grade fuel ethanol. However, this azeotropic distillation is recognized as an energy-consuming process which is contradiction to the concept of bio-ethanol employment for energy saving. Membrane separation as an alternative dehydrating process has been developed to replace the azeotropic distillation. The NaA zeolite membrane for industrial purpose has been developed to be installed in dehydration processes because of its higher permeation flux and higher thermal, chemical and mechanical stabilities than those in organic membranes [1–7]. Actually, the NaA zeolite membranes have advantage of higher permeation fluxes and possibility of utilization under higher pressures and temperatures feed conditions up to 140 °C and 570 kPa for industrial purpose [3,5,7]. The NaA zeolite membranes are expected to be utilized in a hybrid distillation/vapor permeation system for bioethanol dehydration. In this system, a fermented hydrous
K. Sato et al. / Microporous and Mesoporous Materials 115 (2008) 184–188
ethanol with 5–10 wt.% ethanol content should be distillated by rectifiers first and then the overhead hydrous ethanol vapor with azeotropic composition (5–10 wt.% water) is fed to the membrane modules. Field tests are required to verify the viability of NaA zeolite membranes that were set in membrane modules for production of anhydrous motor fuel grade ethanol in a pilot plant in which an actual fermented hydrous ethanol is dealt. In this background, the purpose of this study is: (1) to determine the elemental membrane dehydration performance in a VP unit, and (2) to report results of a field test in a pilot bio-ethanol producing plant to verify the success of anhydrous ethanol production in an industrial scale. 2. Experimental
16φ membrane 19φ
permeate
feed 1m retentate Fig. 1. Schematic diagram for the double-pipe type membrane module. The sealed tubular membrane sample (16 mm O.D.) was set in the metallic tubular module (19 mm I.D.).
→ feed vapor →
2.2. Apparatus for vapor permeation A piece of tubular NaA zeolite membrane (16 mm O.D.) was set in a double-pipe type module formed by stainless tube (19 mm I.D.) (Fig. 1). The vapor permeation experiments were carried out with a semi-pilot scale unit equipped with 25 kW heater, 35 l feed tank, where four modules were arranged in series connection. A schematic layout of the unit is shown in Fig. 2. The feed vapor was
Membrane modules
Feed vessel
permeate
2.1. Membranes preparation The membranes utilized in this study are commercial, tubular NaA zeolite membrane with 1 m long with effective membrane area of ca 460 cm2 per one piece supplied by our company of Bussan Nanotech Research Institute Inc, Japan. The membranes were synthesized hydrothermally on the surface of tubular porous substrate at 80 °C with a low viscous solution whose chemical composition was optimized in the range of Al2O3:SiO2:Na2O:H2O =1: 2– 5:2–50:500–1000. The utilized supports were porous alumina tube and the average pore size was 0.8 lm. A secondary growth method including seeding and hydrothermal treatment [5] was employed in the mass-production. The seed crystals were prepared by wet milling and the those sizes were smaller than 1.0 lm [5]. The significant features in this manufacturing method were: (1) a low viscous solution was employed for hydrothermal synthesis and (2) synthesis temperature was 80 °C with lower synthetic temperatures than those in other studies [1–5]. These features were optimized for handling with ease in synthesis process of mass-production. The membrane was identified with X-ray diffractometer (Rigaku, RINT-Ultima III) and the morphology of this commercial membrane was observed with scanning electron microscopy (SEM, Keyence VE-7800). The quality of the membrane was investigated on a part of the membrane with 10 cm long by pervaporation (PV) at 75 °C in a mixture of water(10 wt.%)/ethanol(90 wt.%) for comparison with those in previous studies. The details in PV experiment were same with our previous study [5].
185
Heater Chiller Product Condenser Circulation Pump Feed Tank Feed Pump
Vacuum Pump Product
Permeate Tank
Fig. 2. Schematic diagram for a semi-pilot scale unit for vapor permeation experiment.
supplied from the feed vessel into the series of membrane modules and then the dehydrated vapor was condensed with a condenser to be liquid state for return to the feed tank. The returned hydrous ethanol was recirculated with a pump. The flow rate of hydrous feed vapor was selected at 3.3, 11 and 17 kg h 1 in this study. These feed flow rates are corresponding to the ethanol retentate producing rates of 3, 10 and 15 kg h 1. The feed temperature was set at 130 °C and vacuum was applied in the permeate side 2– 8 kPa. The membrane performance was represented by the compositional evolution of retentate with proceeding of dehydration. The compositional evolution was determined by chemical analysis of retentates that were sampled at each sampling port of membrane modules (Fig. 3). Furthermore, the membrane performance was evaluated by determination of permeation flux (Q: kg m 2 h 1) and water permeances (Pw: mol m 2 s 1 Pa 1) from the experimental data of retentate composition and mass balance calculation. The chemical analysis on the retentate was performed with a gas chromatography (Shimadzu, GC14B). 3. Results and discussion 3.1. Characterization of the commercial NaA zeolite membranes for this study The results of XRD experiment showed that the NaA zeolite membrane was composed of phases of NaA zeolite
186
K. Sato et al. / Microporous and Mesoporous Materials 115 (2008) 184–188
SP1 permeate1 module1
SP2
permeate2 module2
SP3
permeate3 module3
membrane is basically composed of the active layer, deposited on the outer surface of an alumina porous substrate. Fig. 5 shows that the results of PV for permeation flux was 5.9 kg m 2 h 1 and separation factor (a) was 9000 with those from previous studies, for comparison. Here, the separation factor (a) is (Pw/Pe)/(Fw/Fe), where Pw/Pe is the weight ratio of water to ethanol in permeate, and Fw/Fe is that in feed. The present commercial NaA zeolite membrane exhibits higher permeation flux than those in previously reported NaA zeolite membranes.
permeate4
SP4 module4
SP5
Fig. 3. Schematic diagram for sampling ports (SP1-5) for determination of compositional evolution of ethanol retentate through a series of four membrane modules.
3.2. Membrane performance by VP at 130 °C Fig. 6 shows the compositional evolutions in retentate with proceeding of dehydration through the series of membrane modules in three feed vapor flow rates of 3.3, 11 and 17 kg h 1 at 130 °C. The final ethanol concentration in the range of 99.25–99.86 wt.% was obtained by passing 5
and a-alumina. Fig. 4 shows a typical SEM image of the commercial NaA zeolite membrane. The surface of membrane was covered with faceted crystals that are intersected each other. The crystals exhibit part of cubic shape grain encompassed by {1 0 0} faces. These indicate that the surface morphology is quite different from that in NaA zeolite membrane synthesized with a sol–gel type solution of Al2O3:SiO2:Na2O:H2O =1:2:2:150 [5]. The sharply faceted crystalline features are observed in the present membrane from the low viscous solution, while the rather rounded and hump-like polycrystalline morphology was observed in the membrane from the gel composition. These morphological difference should be caused by primary different crystallization environment of temperature and synthetic solution, compared with the previous study [5]. The SEM image for membrane section indicates that the thickness of active zeolite layer is ca. 10 lm, indicating the tubular
Separation factor, α
10
b c 4
10
a PV at 75ºC
water(10 wt.%)/ethanol(90 wt.%) 3
10
0
2
4
6
8
Permeation flux [kg m-2 h-1] Fig. 5. The membrane performance by PV at 75 °C in a feed mixture of water(10 wt%)/ethanol(90 wt%) for the commercial NaA zeolite membrane (d) investigated in this study with comparison of previously reported higher performance NaA zeolite membranes for industrial purpose and its pioneering study of Ref. [2] (ja), Ref. [3] (jb), Ref. [5] (jc).
Fig. 4. SEM images of the NaA zeolite membrane for surface (a) and section (b).
K. Sato et al. / Microporous and Mesoporous Materials 115 (2008) 184–188
10-5
Membrane number 2nd
3rd
Dehydration
4th
99.86wt.% 99.63wt.% 99.25wt.%
Ethanol concentration in retentate [wt.%]
100 98
3.3 kgh-1 11 kgh-1 17 kgh-1
96 94
Water permeance [mol m-2 sec-1 Pa-1]
1st
187
Re:14000
10
Re:8200
17kgh-1
Re:2400
11kgh-1
-6
3.3 kgh-1
92
10-7 90
Proceeding of dehydration
1st
2nd
3rd
4th
Membrane module 88
SP1
SP2
SP3
SP5
SP4
Position of sampling port Fig. 6. The compositional evolution in ethanol retentate with proceeding of dehydrating at the three feed vapor rates of 3.3, 11 and 17 kg h 1 at 130 °C.
through the series of four membranes with the effective membrane area of ca. 1800 cm2. The ethanol concentration depended on the feed vapor flow rate, indicating that the higher quality retentate can be gained at lower feed rate. It is noted that the flow rate of hydrous vapor was tightly correlated to productivity of ethanol retentate. Therefore, these results indicate that the productivity and the retentate quality is in a trade-off relation. The permeation parameters of flux (kg m 2 h 1) and water permeances (mol m 2 s 1 Pa 1) are shown in Figs. 7 and 8, respectively. Higher permeation fluxes are found in dehydration with higher vapor flow rate as shown by that the flux is decreased from 20 kg m 2 h 1 through 15 kg m 2 h 1 down to 4.6 kg m 2 h 1 for individual dehydration in the first module with decreasing of vapor feed flow rate from 17 kg h 1, 11 kg h 1 to 3.3 kg h 1. The lower permeation flux at a lower feed vapor flow rate
Fig. 8. The water permeance in each membrane module for three dehydrating runs with feed rate of 3, 10 kg h 1 and 15 kg h 1 at 130 °C with the Reynolds number.
should be attributed to concentration polarization on the membrane surface. It is apparent that the increasing of the Reynolds number representing flow conditions on the membrane surface enhanced the increasing of permeation fluxes (Fig. 7). Fig. 8 shows the relations of water permeance and proceeding of dehydration or ethanol content in feed. The water permeance for each dehydration at the given feed vapor flow rate are rather constant up to the third module, indicating that permeation fluxes are controlled by the driving force of water vapor pressure across the membrane.
ethanol 88 wt.% 130ºC, 550 kPa ethanol
30-40 wt.% High pressure distillation tower
Dehydration
Estimated permeation flux [kg m-2 h-1]
membrane module MM1
MM2
at 130ºC
Re:14000
3.3 kgh-1 11 kgh-1 17 kgh-1
Re:8200
Feed tank Vacuum Pump bottom water
Re:2400
1st
2nd
3rd
4th
Membrane module Fig. 7. The permeation fluxes at each membrane from the 1st to 4th for three vapor feed rate conditions at 130 °C with the Reynolds number (Re) on the membrane surface.
ethanol 99.6
wt.%
Retentate tank
Fig. 9. Schematic flow diagram for part of pilot plant. The distillation and membrane separation processes is depicted with ethanol concentration of retentate at each stage.
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K. Sato et al. / Microporous and Mesoporous Materials 115 (2008) 184–188
Fig. 10. Tubular membranes and module (a) and membrane modules in the pilot plant for anhydrous ethanol >99.6 wt% from sugarcane fermentation (b).
On the other hand, permeances decreasing was observed in all three cases, indicating that the reduction of water permeances might occur in the water depleted feed condition. 3.3. Field test in the pilot ethanol producing plant The commercial NaA zeolite membranes were applied to produce anhydrous ethanol with >99.6 wt% from a fermented hydrous ethanol in a pilot plant including hybrid distillation/vapor permeation system. Fig. 9 shows the diagram for part of distillation and vapor permeation processes. The tubular membranes were set in the membrane modules having double-tube type structure for all individual membrane (Fig. 10a) and two modules were installed in the pilot plant (Fig. 10b). The hydrous ethanol with 30–40 wt% ethanol content is distillated in the distillation tower to provide more condensed hydrous ethanol (88 wt%) vapor with 130 °C and 550 kPa to the membrane modules being arranged in series. The retentate vapor that is finally purified through the membrane modules is condensed to be stored in the retentate tank. It has been clarified that the amount of 1100 kg anhydrous ethanol with >99.6 wt% was stably produced by continuous operation for 24 h in the pilot plant from beer containing 6–7.4 wt% ethanol from fermentation of sugarcane. The beer with 6.0–7.4 wt% ethanol was distillated through three stages to supply the hydrous vapor with 88 wt% ethanol into the first zeolite membrane module. The retentate with 99.3 wt% ethanol was observed in the output at the first module and then the final ethanol retentate with >99.6 wt% was obtained. These results indicate that the commercial NaA zeolite membranes can be applied to anhydrous ethanol production.
4. Conclusions The commercial NaA zeolite membranes from Bussan Nanotech Research Institute Inc., Japan were investigated to clarify their membrane performance by vapor permeation at 130 °C in ethanol dehydration in a semi-pilot plant scale unit. The high membrane performance with permeation fluxes of 20 kg m 2 h 1 was observed at 130 °C in a feed mixture of water(10 wt%)/ethanol(90 wt%). Furthermore, a field test was performed using the commercial NaA zeolite membranes to produce anhydrous ethanol from fermented beer from sugarcane. It has been verified that the amount of 1100 kg anhydrous ethanol with >99.6 wt% could be produced for 24 h continuous operation. Acknowledgment This research was financially supported by the Ministry of the Environment (MOE), Government of Japan. References [1] H. Kita, K. Horii, Y. Ohtoshi, K. Tanaka, K. Okamoto, J. Mater. Sci. Lett. 14 (1995) 206. [2] K. Okamoto, H. Kita, K. Horii, K. Tanaka, M. Kondo, Ind. Eng. Chem. Res. 40 (2001) 163. [3] M. Kondo, M. Komori, H. Kita, K.-I. Okamoto, J. Membr. Sci. 133 (1997) 133. [4] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, Sep. Purif. Technol. 25 (2001) 251. [5] K. Sato, T. Nakane, J. Membr. Sci. 301 (2007) 151. [6] K. Sato, S. Ikeda, J. Saito, T. Sawasaki, T. Kyotani, S. Kakui, T. Shimamoto, R. Sato, T. Nakane, in: Proceeding of China/USA/Japan Joint Chemical Engineering Conference, Beijing, 2005, pp. 80–81. [7] H. Richter, I. Voigt, J.-T. Kuhnert, Desalination 199 (2006) 92.
Microporous and Mesoporous Materials 115 (2008) 184–188 www.elsevier.com/locate/micromeso
Dehydrating performance of commercial LTA zeolite membranes and application to fuel grade bio-ethanol production by hybrid distillation/vapor permeation process Kiminori Sato *, Katsuhiko Aoki, Kazunori Sugimoto, Kou Izumi, Soushi Inoue, Junji Saito, Shiro Ikeda, Takashi Nakane Bussan Nanotech Research Institute Inc., Koyadai 2-1, 305-0074 Tsukuba, Japan Received 19 July 2007; accepted 24 October 2007 Available online 19 February 2008
Abstract The performance of commercial NaA zeolite membranes (Bussan Nanotech Research Institute Inc., Japan) was investigated for dehydration viability by vapor permeation (VP) to apply them to production of fuel grade ethanol. We carried out mainly two sub-projects: (1) the VP experiments with a large unit to determine the dehydration performance for elemental membranes and (2) the field test to produce fuel grade ethanol from fermented hydrous ethanol (5–10 wt.% ethanol content) through hybrid distillation/vapor permeation system in which the membrane modules were installed. The commercial NaA zeolite membranes exhibited sufficiently high dehydrating performance for economically feasible fluxes. It has been also verified that anhydrous ethanol (>99.7 wt.%) with an amount of 46 kg h 1 can be continuously produced with the commercial NaA zeolite membranes in a pilot plant. These results suggest that the commercial NaA zeolite membrane can be expected to be utilized in motor fuel ethanol production. Ó 2008 Published by Elsevier Inc. Keywords: LTA; Zeolite membrane; Bio-ethanol; Dehydration; Vapor permeation
1. Introduction Ethanol production for motor fuel grade is rapidly growing worldwide, because it has been recommended by governments to utilize bio-ethanol as an additive to gasoline for reduction of fossil energy consumption and of CO2 emission. Many kinds of feedstock are fermented to produce hydrous ethanol that has to be purified finally to the anhydrous ethanol (>99.2 wt.%) by dehydration for motor grade fuel. Distillation is a dehydration process for hydrous ethanol, but it is well known that simple distillation process cannot purify to anhydrous ethanol because there is the azeotrope with water(4 wt.%)/ethanol(96 wt.%) through which more anhydrous ethanol cannot be produced. The azeotropic distillation by additional component *
Corresponding author. Tel.: +81 29 839 9371; fax: +81 29 29 9430. E-mail address: [email protected] (K. Sato).
1387-1811/$ - see front matter Ó 2008 Published by Elsevier Inc. doi:10.1016/j.micromeso.2007.10.053
is one way to break the azeotrope for purification of motor grade fuel ethanol. However, this azeotropic distillation is recognized as an energy-consuming process which is contradiction to the concept of bio-ethanol employment for energy saving. Membrane separation as an alternative dehydrating process has been developed to replace the azeotropic distillation. The NaA zeolite membrane for industrial purpose has been developed to be installed in dehydration processes because of its higher permeation flux and higher thermal, chemical and mechanical stabilities than those in organic membranes [1–7]. Actually, the NaA zeolite membranes have advantage of higher permeation fluxes and possibility of utilization under higher pressures and temperatures feed conditions up to 140 °C and 570 kPa for industrial purpose [3,5,7]. The NaA zeolite membranes are expected to be utilized in a hybrid distillation/vapor permeation system for bioethanol dehydration. In this system, a fermented hydrous
K. Sato et al. / Microporous and Mesoporous Materials 115 (2008) 184–188
ethanol with 5–10 wt.% ethanol content should be distillated by rectifiers first and then the overhead hydrous ethanol vapor with azeotropic composition (5–10 wt.% water) is fed to the membrane modules. Field tests are required to verify the viability of NaA zeolite membranes that were set in membrane modules for production of anhydrous motor fuel grade ethanol in a pilot plant in which an actual fermented hydrous ethanol is dealt. In this background, the purpose of this study is: (1) to determine the elemental membrane dehydration performance in a VP unit, and (2) to report results of a field test in a pilot bio-ethanol producing plant to verify the success of anhydrous ethanol production in an industrial scale. 2. Experimental
16φ membrane 19φ
permeate
feed 1m retentate Fig. 1. Schematic diagram for the double-pipe type membrane module. The sealed tubular membrane sample (16 mm O.D.) was set in the metallic tubular module (19 mm I.D.).
→ feed vapor →
2.2. Apparatus for vapor permeation A piece of tubular NaA zeolite membrane (16 mm O.D.) was set in a double-pipe type module formed by stainless tube (19 mm I.D.) (Fig. 1). The vapor permeation experiments were carried out with a semi-pilot scale unit equipped with 25 kW heater, 35 l feed tank, where four modules were arranged in series connection. A schematic layout of the unit is shown in Fig. 2. The feed vapor was
Membrane modules
Feed vessel
permeate
2.1. Membranes preparation The membranes utilized in this study are commercial, tubular NaA zeolite membrane with 1 m long with effective membrane area of ca 460 cm2 per one piece supplied by our company of Bussan Nanotech Research Institute Inc, Japan. The membranes were synthesized hydrothermally on the surface of tubular porous substrate at 80 °C with a low viscous solution whose chemical composition was optimized in the range of Al2O3:SiO2:Na2O:H2O =1: 2– 5:2–50:500–1000. The utilized supports were porous alumina tube and the average pore size was 0.8 lm. A secondary growth method including seeding and hydrothermal treatment [5] was employed in the mass-production. The seed crystals were prepared by wet milling and the those sizes were smaller than 1.0 lm [5]. The significant features in this manufacturing method were: (1) a low viscous solution was employed for hydrothermal synthesis and (2) synthesis temperature was 80 °C with lower synthetic temperatures than those in other studies [1–5]. These features were optimized for handling with ease in synthesis process of mass-production. The membrane was identified with X-ray diffractometer (Rigaku, RINT-Ultima III) and the morphology of this commercial membrane was observed with scanning electron microscopy (SEM, Keyence VE-7800). The quality of the membrane was investigated on a part of the membrane with 10 cm long by pervaporation (PV) at 75 °C in a mixture of water(10 wt.%)/ethanol(90 wt.%) for comparison with those in previous studies. The details in PV experiment were same with our previous study [5].
185
Heater Chiller Product Condenser Circulation Pump Feed Tank Feed Pump
Vacuum Pump Product
Permeate Tank
Fig. 2. Schematic diagram for a semi-pilot scale unit for vapor permeation experiment.
supplied from the feed vessel into the series of membrane modules and then the dehydrated vapor was condensed with a condenser to be liquid state for return to the feed tank. The returned hydrous ethanol was recirculated with a pump. The flow rate of hydrous feed vapor was selected at 3.3, 11 and 17 kg h 1 in this study. These feed flow rates are corresponding to the ethanol retentate producing rates of 3, 10 and 15 kg h 1. The feed temperature was set at 130 °C and vacuum was applied in the permeate side 2– 8 kPa. The membrane performance was represented by the compositional evolution of retentate with proceeding of dehydration. The compositional evolution was determined by chemical analysis of retentates that were sampled at each sampling port of membrane modules (Fig. 3). Furthermore, the membrane performance was evaluated by determination of permeation flux (Q: kg m 2 h 1) and water permeances (Pw: mol m 2 s 1 Pa 1) from the experimental data of retentate composition and mass balance calculation. The chemical analysis on the retentate was performed with a gas chromatography (Shimadzu, GC14B). 3. Results and discussion 3.1. Characterization of the commercial NaA zeolite membranes for this study The results of XRD experiment showed that the NaA zeolite membrane was composed of phases of NaA zeolite
186
K. Sato et al. / Microporous and Mesoporous Materials 115 (2008) 184–188
SP1 permeate1 module1
SP2
permeate2 module2
SP3
permeate3 module3
membrane is basically composed of the active layer, deposited on the outer surface of an alumina porous substrate. Fig. 5 shows that the results of PV for permeation flux was 5.9 kg m 2 h 1 and separation factor (a) was 9000 with those from previous studies, for comparison. Here, the separation factor (a) is (Pw/Pe)/(Fw/Fe), where Pw/Pe is the weight ratio of water to ethanol in permeate, and Fw/Fe is that in feed. The present commercial NaA zeolite membrane exhibits higher permeation flux than those in previously reported NaA zeolite membranes.
permeate4
SP4 module4
SP5
Fig. 3. Schematic diagram for sampling ports (SP1-5) for determination of compositional evolution of ethanol retentate through a series of four membrane modules.
3.2. Membrane performance by VP at 130 °C Fig. 6 shows the compositional evolutions in retentate with proceeding of dehydration through the series of membrane modules in three feed vapor flow rates of 3.3, 11 and 17 kg h 1 at 130 °C. The final ethanol concentration in the range of 99.25–99.86 wt.% was obtained by passing 5
and a-alumina. Fig. 4 shows a typical SEM image of the commercial NaA zeolite membrane. The surface of membrane was covered with faceted crystals that are intersected each other. The crystals exhibit part of cubic shape grain encompassed by {1 0 0} faces. These indicate that the surface morphology is quite different from that in NaA zeolite membrane synthesized with a sol–gel type solution of Al2O3:SiO2:Na2O:H2O =1:2:2:150 [5]. The sharply faceted crystalline features are observed in the present membrane from the low viscous solution, while the rather rounded and hump-like polycrystalline morphology was observed in the membrane from the gel composition. These morphological difference should be caused by primary different crystallization environment of temperature and synthetic solution, compared with the previous study [5]. The SEM image for membrane section indicates that the thickness of active zeolite layer is ca. 10 lm, indicating the tubular
Separation factor, α
10
b c 4
10
a PV at 75ºC
water(10 wt.%)/ethanol(90 wt.%) 3
10
0
2
4
6
8
Permeation flux [kg m-2 h-1] Fig. 5. The membrane performance by PV at 75 °C in a feed mixture of water(10 wt%)/ethanol(90 wt%) for the commercial NaA zeolite membrane (d) investigated in this study with comparison of previously reported higher performance NaA zeolite membranes for industrial purpose and its pioneering study of Ref. [2] (ja), Ref. [3] (jb), Ref. [5] (jc).
Fig. 4. SEM images of the NaA zeolite membrane for surface (a) and section (b).
K. Sato et al. / Microporous and Mesoporous Materials 115 (2008) 184–188
10-5
Membrane number 2nd
3rd
Dehydration
4th
99.86wt.% 99.63wt.% 99.25wt.%
Ethanol concentration in retentate [wt.%]
100 98
3.3 kgh-1 11 kgh-1 17 kgh-1
96 94
Water permeance [mol m-2 sec-1 Pa-1]
1st
187
Re:14000
10
Re:8200
17kgh-1
Re:2400
11kgh-1
-6
3.3 kgh-1
92
10-7 90
Proceeding of dehydration
1st
2nd
3rd
4th
Membrane module 88
SP1
SP2
SP3
SP5
SP4
Position of sampling port Fig. 6. The compositional evolution in ethanol retentate with proceeding of dehydrating at the three feed vapor rates of 3.3, 11 and 17 kg h 1 at 130 °C.
through the series of four membranes with the effective membrane area of ca. 1800 cm2. The ethanol concentration depended on the feed vapor flow rate, indicating that the higher quality retentate can be gained at lower feed rate. It is noted that the flow rate of hydrous vapor was tightly correlated to productivity of ethanol retentate. Therefore, these results indicate that the productivity and the retentate quality is in a trade-off relation. The permeation parameters of flux (kg m 2 h 1) and water permeances (mol m 2 s 1 Pa 1) are shown in Figs. 7 and 8, respectively. Higher permeation fluxes are found in dehydration with higher vapor flow rate as shown by that the flux is decreased from 20 kg m 2 h 1 through 15 kg m 2 h 1 down to 4.6 kg m 2 h 1 for individual dehydration in the first module with decreasing of vapor feed flow rate from 17 kg h 1, 11 kg h 1 to 3.3 kg h 1. The lower permeation flux at a lower feed vapor flow rate
Fig. 8. The water permeance in each membrane module for three dehydrating runs with feed rate of 3, 10 kg h 1 and 15 kg h 1 at 130 °C with the Reynolds number.
should be attributed to concentration polarization on the membrane surface. It is apparent that the increasing of the Reynolds number representing flow conditions on the membrane surface enhanced the increasing of permeation fluxes (Fig. 7). Fig. 8 shows the relations of water permeance and proceeding of dehydration or ethanol content in feed. The water permeance for each dehydration at the given feed vapor flow rate are rather constant up to the third module, indicating that permeation fluxes are controlled by the driving force of water vapor pressure across the membrane.
ethanol 88 wt.% 130ºC, 550 kPa ethanol
30-40 wt.% High pressure distillation tower
Dehydration
Estimated permeation flux [kg m-2 h-1]
membrane module MM1
MM2
at 130ºC
Re:14000
3.3 kgh-1 11 kgh-1 17 kgh-1
Re:8200
Feed tank Vacuum Pump bottom water
Re:2400
1st
2nd
3rd
4th
Membrane module Fig. 7. The permeation fluxes at each membrane from the 1st to 4th for three vapor feed rate conditions at 130 °C with the Reynolds number (Re) on the membrane surface.
ethanol 99.6
wt.%
Retentate tank
Fig. 9. Schematic flow diagram for part of pilot plant. The distillation and membrane separation processes is depicted with ethanol concentration of retentate at each stage.
188
K. Sato et al. / Microporous and Mesoporous Materials 115 (2008) 184–188
Fig. 10. Tubular membranes and module (a) and membrane modules in the pilot plant for anhydrous ethanol >99.6 wt% from sugarcane fermentation (b).
On the other hand, permeances decreasing was observed in all three cases, indicating that the reduction of water permeances might occur in the water depleted feed condition. 3.3. Field test in the pilot ethanol producing plant The commercial NaA zeolite membranes were applied to produce anhydrous ethanol with >99.6 wt% from a fermented hydrous ethanol in a pilot plant including hybrid distillation/vapor permeation system. Fig. 9 shows the diagram for part of distillation and vapor permeation processes. The tubular membranes were set in the membrane modules having double-tube type structure for all individual membrane (Fig. 10a) and two modules were installed in the pilot plant (Fig. 10b). The hydrous ethanol with 30–40 wt% ethanol content is distillated in the distillation tower to provide more condensed hydrous ethanol (88 wt%) vapor with 130 °C and 550 kPa to the membrane modules being arranged in series. The retentate vapor that is finally purified through the membrane modules is condensed to be stored in the retentate tank. It has been clarified that the amount of 1100 kg anhydrous ethanol with >99.6 wt% was stably produced by continuous operation for 24 h in the pilot plant from beer containing 6–7.4 wt% ethanol from fermentation of sugarcane. The beer with 6.0–7.4 wt% ethanol was distillated through three stages to supply the hydrous vapor with 88 wt% ethanol into the first zeolite membrane module. The retentate with 99.3 wt% ethanol was observed in the output at the first module and then the final ethanol retentate with >99.6 wt% was obtained. These results indicate that the commercial NaA zeolite membranes can be applied to anhydrous ethanol production.
4. Conclusions The commercial NaA zeolite membranes from Bussan Nanotech Research Institute Inc., Japan were investigated to clarify their membrane performance by vapor permeation at 130 °C in ethanol dehydration in a semi-pilot plant scale unit. The high membrane performance with permeation fluxes of 20 kg m 2 h 1 was observed at 130 °C in a feed mixture of water(10 wt%)/ethanol(90 wt%). Furthermore, a field test was performed using the commercial NaA zeolite membranes to produce anhydrous ethanol from fermented beer from sugarcane. It has been verified that the amount of 1100 kg anhydrous ethanol with >99.6 wt% could be produced for 24 h continuous operation. Acknowledgment This research was financially supported by the Ministry of the Environment (MOE), Government of Japan. References [1] H. Kita, K. Horii, Y. Ohtoshi, K. Tanaka, K. Okamoto, J. Mater. Sci. Lett. 14 (1995) 206. [2] K. Okamoto, H. Kita, K. Horii, K. Tanaka, M. Kondo, Ind. Eng. Chem. Res. 40 (2001) 163. [3] M. Kondo, M. Komori, H. Kita, K.-I. Okamoto, J. Membr. Sci. 133 (1997) 133. [4] Y. Morigami, M. Kondo, J. Abe, H. Kita, K. Okamoto, Sep. Purif. Technol. 25 (2001) 251. [5] K. Sato, T. Nakane, J. Membr. Sci. 301 (2007) 151. [6] K. Sato, S. Ikeda, J. Saito, T. Sawasaki, T. Kyotani, S. Kakui, T. Shimamoto, R. Sato, T. Nakane, in: Proceeding of China/USA/Japan Joint Chemical Engineering Conference, Beijing, 2005, pp. 80–81. [7] H. Richter, I. Voigt, J.-T. Kuhnert, Desalination 199 (2006) 92.